1. Field of the Invention
The present invention relates to a focus adjustment technique using contrast information obtained from an image sensor.
2. Description of the Related Art
In a digital camera configured to acquire digital data of an image formed by a photographic lens, auto-focus (hereinafter, referred to as “AF”) control information is generally obtained from an image sensor including photoelectric conversion elements disposed in a two-dimensional pattern. This kind of image sensor is, for example, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Furthermore, automatic exposure (hereinafter, referred to as “AE”) control information and white balance (hereinafter, referred to as “WB”) control information are obtained from the image sensor.
A technique generally referred to as “contrast” (or “hill-climbing” or “TV-AF”) method is applicable to the AF control operation. The “contrast” method includes moving a focusing lens of a photographic lens unit, or an image sensor, in the optical axis direction, then acquiring an evaluation value representing the contrast of a captured image obtained in each driving stage, and identifying a lens position where the evaluation value is maximized as an in-focus position, as discussed in Japanese Patent No. 02821214.
If the AF control operation is performed using contrast information of the entire image plane, perspective competition among a plurality of objects different in shooting distance adversely influences the AF control operation. Therefore, the contrast AF control operation is usually performed on a limited part of the image plane, which can be selected automatically or manually.
In general, a digital single-lens reflex camera includes a mirror or a shutter configured to prevent an image sensor from being irradiated with light when the image sensor is not used for an exposure operation. Accordingly, an AF sensor and an AE sensor (additional sensors dedicated to the AF and AE control operations) are installed on the digital single-lens reflex camera and configured to receive part of light entered from a photographic lens, which is guided with an optical path division mechanism.
The digital single-lens reflex cameras are recently required to perform a live view operation for displaying an image captured by an image sensor on a rear liquid crystal panel or a similar display element to enable a user to confirm the composition or a focused state. In the live view operation, a mirror rotates upward and a shutter opens to bring the image sensor into an exposure state. In this state, the AF sensor and the AE sensor cannot receive any light, and therefore the AF and AE control operations based on these dedicated sensors cannot be realized. Hence, the contrast AF and imaging plane AE control operations using the image sensor are required to perform the AF and AE control operations during the live view operation.
The contrast AF control operation performed by a single-lens reflex camera includes obtaining a contrast evaluation value while driving a lens. In this case, in addition to the obtained contrast evaluation value, information relating to a lens position where the evaluation value was acquired is required to obtain an in-focus position.
In some single-lens reflex cameras each capable of performing an AF operation, a focusing lens driving motor is installed on an interchangeable lens and a lens driving command is transmitted from a camera to the lens.
In such a single-lens reflex camera system, lens position information is acquired from a pulse encoder provided in the interchangeable lens. The camera can receive the lens position information transmitted from the lens.
As discussed in Japanese Patent Application Laid-Open No. 2002-131621, there is a phase-difference AF sensor capable of synchronizing accumulation timing of the AF sensor with lens position information.
In the contrast AF control operation, as described above, a contrast evaluation value and information relating to a lens position where the evaluation value was acquired are required to obtain an in-focus position.
The contrast evaluation value can be obtained for each image accumulated by the image sensor, i.e., for each frame. Therefore, obtaining lens position information corresponding to each frame is desired.
To perform the contrast AF control operation speedily and accurately, it is necessary to move the lens quickly or increase the number of times of acquiring the contrast evaluation value.
However, according to a system including a pulse encoder provided in a lens, lens position information obtained by the pulse encoder is transmitted to a camera using communication between the lens and the camera. Therefore, the amount of communications performed between the lens and the camera increases if the number of times of acquiring the contrast evaluation value, i.e., the frame rate in the AF operation, is increased.